1. Field of the Invention
The present invention relates in general to the guidance of a homing missile toward a target. More specifically, the invention is directed to a seeker circuit for passive homing missile guidance. The seeker circuit uses estimated line-of-sight rate components of the last actual line-of-sight rate during the absence of a target signal.
2. Description of Prior Art
Missile guidance performance is a function of the rate at which target data is derived. Missile guidance generally requires a data rate commensurate with the bandwidth of its guidance loop. An inadequate data rate is detrimental to guidance accuracy.
Inadequate data rates may be caused by a number of factors including, but not limited to the following: interference from non-target radar signals, jamming techniques, data starving periodic cessation of radar transmission from the target, and sudden cool spots (for heat seeking guidance systems). Such detriments to the desired data signal may irreparably effect guidance allowing the missile to veer off course.
Data starving is a common means for defending against a missile guidance system. Target data is intermittently denied to the missile by periodically ceasing radar transmission from the target. If the target radiation duty cycle is sufficiently low, the missile is unable to receive target data at a rate required for accurate missile guidance.
To overcome a data starving defense (or any other detriment to the data signal), conventional homing missile guidance systems, and more particularly, missile seekers employ "velocity memory" to coast through periods when target data is being denied. The missile seekers compute velocity components of the last line-of-sight (LOS) signal input prior to data starvation to maintain an approximately accurate course. These velocity components of the line-of-sight are important to guidance because homing missiles commonly use proportional navigation guidance wherein the missile's turning rate is made proportional to the turning rate of the line-of-sight between the missile and its target.
As shown in FIG. 1 (PRIOR ART), a conventional seeker comprises a target detector 30 and a velocity memory loop 10 including an error detection circuit 26, a switch 24, a first integrator 12 and a second integrator 14.
When the target detector 30 detects an adequate data signal, it controls first switch 24 to terminal A. The error detection circuit 26 produces a first error signal 22, which is proportional to the difference between an actual line-of-sight 18 and an estimated line-of-sight 16. The first integrator 12 integrates first error signal 22 to generate a commanded LOS rate signal 20, which is effectively the rate at which the line-of-sight is turning. The commanded LOS rate signal 20 is input to a guidance computer of the missile and is used to drive a missile antenna (not shown) to move along the line-of-sight. To complete the loop, the second integrator 14 further integrates the commanded LOS rate signal 20 to provide the estimated line-of-sight 16 which is fed back to the error detection circuit 26.
During the absence of an adequate data signal (i.e. any detriment previously discussed), target detector 30 controls the first switch 24 to ground terminal B. Thus, the first error signal 22 is forced to zero causing the first integrator 12 to stop integrating. As a result, the last integrated commanded line-of-sight rate signal 20 is preserved, or "memorized" (hence, the name "velocity memory"). This allows the second integrator 14 to continue integrating from this preserved value despite a loss of an adequate data signal.
However, velocity memory is limited in that it only memorizes velocity. Yet, the last line-of-sight may have acceleration components in addition to velocity components. Further, velocity memory has a limited capability of holding an accurate course during longer intervals between adequate signals.